[marketplace, proofs] simplify proofTimeout getter

contracts/Marketplace.sol

@@ -308,10 +308,6 @@ contract Marketplace is Collateral, Proofs, StateRetrieval {
     return secondsPerPeriod;
   }
 
-  function proofTimeout() public view returns (uint256) {
-    return _timeout();
-  }
-
   function proofEnd(SlotId slotId) public view override returns (uint256) {
     return requestEnd(_slot(slotId).requestId);
   }

contracts/Proofs.sol

@@ -5,7 +5,7 @@ import "./Requests.sol";
 import "./Periods.sol";
 
 abstract contract Proofs is Periods {
-  uint256 private immutable timeout;
+  uint256 public immutable proofTimeout;
   uint8 private immutable downtime;
 
   constructor(
@@ -14,7 +14,7 @@ abstract contract Proofs is Periods {
     uint8 __downtime
   ) Periods(__period) {
     require(block.number > 256, "Insufficient block height");
-    timeout = __timeout;
+    proofTimeout = __timeout;
     downtime = __downtime;
   }
 
@@ -25,10 +25,6 @@ abstract contract Proofs is Periods {
   mapping(SlotId => mapping(Period => bool)) private received;
   mapping(SlotId => mapping(Period => bool)) private missing;
 
-  function _timeout() internal view returns (uint256) {
-    return timeout;
-  }
-
   // Override this to let the proving system know when proofs for a
   // slot are no longer required.
   function proofEnd(SlotId id) public view virtual returns (uint256);
@@ -130,7 +126,7 @@ abstract contract Proofs is Periods {
   function _markProofAsMissing(SlotId id, Period missedPeriod) internal {
     uint256 periodEnd = periodEnd(missedPeriod);
     require(periodEnd < block.timestamp, "Period has not ended yet");
-    require(block.timestamp < periodEnd + timeout, "Validation timed out");
+    require(block.timestamp < periodEnd + proofTimeout, "Validation timed out");
     require(!received[id][missedPeriod], "Proof was submitted, not missing");
     require(_isProofRequired(id, missedPeriod), "Proof was not required");
     require(!missing[id][missedPeriod], "Proof already marked as missing");

contracts/TestProofs.sol

@@ -22,10 +22,6 @@ contract TestProofs is Proofs {
     return ends[slotId];
   }
 
-  function timeout() public view returns (uint256) {
-    return _timeout();
-  }
-
   function startRequiringProofs(SlotId slot, uint256 _probability) public {
     _startRequiringProofs(slot, _probability);
   }